Electroplating and electroless plating can be used for the deposition of continuous metal layers as well as patterned metal layers. Electroless plating is often used as a preliminary step in preparing a seed layer for conventional electroplating. One of the process sequences used by the microelectronic manufacturing industry to deposit metals onto semiconductor wafers is known to as a “damascene” process. In such a process, via openings, trenches and/or other recesses, are formed in a dielectric layer and filled with a metal, such as copper. The wafer, with vias and trenches etched in the dielectric material, first receives a metallic seed layer, which is used to conduct electrical currents during a subsequent metal electroplating step. If a metal such as copper is used, the seed layer is disposed over a barrier layer material, such as Ti, TiN, etc. The seed layer is a very thin layer of metal typically formed by electroless plating. The subsequent plating on the seed layer is typically electroplating.
FIG. 1 illustrates a conventional apparatus for electroless plating. Wafer 2 is placed on wafer holder 4, which includes guide pins 6 for limiting wafer 2. The chemical dispensing nozzle 8, that dispenses plating chemicals, is connected to a chemical dispenser (not shown). Typically, Electroless plating is performed at elevated temperatures by conducting hot de-ionized (DI) water under wafer 2, wherein wafer 2 may be in direct contact with the DI water. In a typical design, the hot DI water is conducted to the bottom center of a wafer, and then spread to the edges, as illustrated by the arrows. In addition, the plating chemicals may be heated before they are dispensed on the surface of wafer 2.
It is known that temperatures affect chemical reactions, and thus the deposition rate in an electroless plating is also sensitive to the temperatures. In certain cases, for example, in the Ni—P electroless plating process, the deposition rate may increase as high as twofold for every 10-degree increase in the plating temperature. Therefore, it is preferred that the temperature at the surface of wafer 2 is uniform. However, in the conventional heating scheme, when hot DI wafer flows from the bottom center of wafer 2 to the edge, due to heat dissipation, the hot DI water may develop an increasingly lower temperature along its path. The edge portions of wafer 2 accordingly have a lower temperature as compared to the center portion. The temperature difference may be as high as 5 degrees centigrade. This causes a significant variation in deposition rates between the center portion and the edge portions of the wafer.
Therefore, an electroless plating apparatus for uniformly heating a wafer and methods for achieving the same are needed.